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IAG Planetary Geomorphology Working Group

Featured images for May 2012:

Surface dissolution on Titan and Earth: Ontario Lacus and the Etosha Pan (Namibia) .

Image and caption contributed by homas.Cornet [at] univ-nantes.fr (Thomas Cornet), Olivier Bourgeois, S. Le Mouélic et al.. Laboratoire de Planétologie et Géodynamique de Nantes, , Université de Nantes, UMR 6112, CNRS, Nantes, France.

Titan, Saturn’s major moon, possesses hydrocarbon lakes and seas in the polar regions [Stofan et al., 2007, Hayes et al., 2008]. Among these, Ontario Lacus (72°S, 180°E, Image 1) is the largest in the south (235 km-long, 75 km-wide). So far it is interpreted as a liquid-covered lake in Titan’s southern hemisphere because of its dark appearance in Cassini image data [Barnes et al., 2009; Turtle et al., 2009; Hayes et al., 2010; Wall et al. 2010], the identification of liquid ethane in its interior [Brown et al., 2008] and the smoothness of its surface [Wye et al., 2009].

 Image 1

Image 1: Ontario Lacus (Titan) and the Etosha Pan (Namibia) as surface dissolution morphologies under arid climates. Credits: Envisat ASAR, data provided by the European Space Agency ©ESA 2009, ESA ®; Cassini RADAR, data provided by JPL/NASA. Link to high resolution image

 

The detailed geomorphological study of Ontario Lacus and its surroundings (Image 2), however revealed the presence of channels inside the southern part of the interpreted liquid-covered depression, seen with multiple sensors at the same location between 2007 and 2010 [Cornet et al., 2012]. This seems to indicate that the southern part of the depression floor was actually not fully liquid-covered at the time of the observations, and was most probably an exposed flat-floor with a substratum saturated in liquids.

Given the extremely flat topographic settings of the region (Image 2) [Wye et al, 2009; Wall et al., 2010], and the presence of channels and small lakes in the flat alluvial plain in which Ontario Lacus lies [Wall et al., 2010; Cornet et al., 2012], an “alkanofer”, (an analog to the groundwater aquifer on Earth) would be located close to the topographic surface, thus inundating topographic lows inside Ontario Lacus’ depression and the surrounding plain.

 

 Image 2

Image 2: Geomorphological map and interpretative cross-section of Ontario Lacus on Titan, compiled using Cassini Visual and Infrared Mapping Spectrometer (VIMS), Imaging Science Subsystem (ISS) and RADAR data. Link to high resolution image

Titan’s climate is characterized by sporadic torrential hydrocarbon rainstorms [Turtle et al., 2011], with expected yearly-averaged evaporation rates greater than yearly-averaged precipitation rates [Mitri et al., 2007]. This balance between evaporation and precipitation is very similar to that in arid/semi-arid areas on Earth, such as in the Owambo Basin in Namibia (Image 1) [Mendelsohn et al., 2002].

In this flat and wide sedimentary basin (Image 3), flat-floored depressions called “pans” develop by dissolution of a surface soluble calcretes layer and the vertical motion of the groundwater table [Miller et al., 2010; Bowen and Johnson, 2012]. The largest pan of the Owambo Basin, namely the Etosha Pan (18°S, 16°E, Image 1), is closely similar in size and shape to Ontario Lacus.

Since solid hydrocarbon compounds that form in Titan’s atmosphere fall onto the surface they can accumulate up to several tens of meters in thickness over geological timescales [Malaska et al., 2011].  These compounds are potentially soluble in liquid methane and ethane [Cordier et al., 2009; Malaska et al., 2011] and suggest that a surface soluble layer could therefore have formed on Titan’s surface. Based on the analogy with the Etosha Pan and on thermochemical models, one possible mechanism to form Ontario Lacus’ depression could be the dissolution of a surface soluble layer on Titan, due to vertical motions of the “alkanofer”.

 

 

 Image 3

Image 3: Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Global Digital Elevation Model data over the Owambo Basin and associated geological cross-section of the Etosha region. ASTER GDEM is a product of METI and NASA. Link to high resolution image

Further Reading:

Barnes, J. W., Brown, R. H., Soderblom, J. M., Soderblom, L. A., Jaumann, R., Jackson, B., Le Mouélic, S., Sotin, C., Buratti, B. J., Pitman, K. M., Baines, K. H., Clark, R. N., Nicholson, P. D., Turtle, E. P., & Perry, J. (2009). Shoreline features of Titan’s Ontario Lacus from Cassini/VIMS observations. Icarus, 201, 217 – 225.

Bowen, M. W., & Johnson, W. C. (2012). Late quaternary environmental reconstructions of playa-lunette system evolution on the central High Plains of Kansas, United States. Geological Society of America Bulletin, 124, 146–161.

Brown, R. H., Soderblom, L. A., Soderblom, J. M., Clark, R. N., Jaumann, R., Barnes, J. W., Sotin, C., Buratti, B., Baines, K. H., & Nicholson, P. D. (2008). The identification of liquid ethane in Titan’s Ontario Lacus. Nature, 454, 607 – 610.

Cordier, D., Mousis, O., Lunine, J. I., Lavvas, P., & Vuitton, V. (2009). An estimate of the chemical composition of Titan’s lakes. The Astrophysical Journal, 707, L128 – L131.

Cornet, T., Bourgeois, O., Le Mouélic, S. Rodriguez, S., Lopez Gonzalez, T., Sotin, C., Tobie, G., Fleurant, C., Barnes, J. W., Brown, R. H., Baines, K. H., Buratti, B. J., Clark, R. N., & Nicholson, P. D. (2012). Geomorphological significance of Ontario Lacus on Titan: Integrated interpretation of Cassini VIMS, ISS and RADAR data and comparison with the Etosha Pan (Namibia). Icarus, Vol. 218, 2, p.788-806. 

Hayes, A. G., Wolf, A. S., Aharonson, O., Zebker, H., Lorenz, R., Kirk, R. L., Paillou, P., Lunine, J., Wye, L., Callahan, P., Wall, S., & Elachi, C. (2010). Bathymetry and absorptivity of Titan’s Ontario Lacus. Journal of Geophysical Research, 115, E09009.Hayes, A., Aharonson, O., Callahan, P., Elachi, C., Gim, Y., Kirk, R., Lewis, K., Lopes, R., Lorenz, R., Lunine, J., Mitchell, K., Mitri, G., Stofan, E., &Wall, S. (2008). Hydrocarbon lakes on Titan: Distribution and interaction with an isotropic porous regolithGeophysical Research Letters, 35, L09204.

Malaska, M., Radebaugh, J., Mitchell, K., Lopes, R., Wall, S. & Lorenz, R. (2011). Surface dissolution model for Titan karst. First International Planetary Cave research Workshop, p. 8018.

Mendelsohn, J., Jarvis, A., Roberts, C., & Robertson, T. (2002). Atlas of Namibia. A portrait of the land and its people. David Phlips Publishers. 200pp.

Mitri, G., Showman, A. P., Lunine, J. I., & Lorenz, R. D. (2007). Hydrocarbon lakes on Titan. Icarus, 186, 385 – 394.

Turtle, E. P., Perry, J. E., Hayes, A. G., Lorenz, R. D., Barnes, J. W., McEwen, A. S., West, R. A., Del Genio, A. D., Barbara, J. M., Lunine, J. I., Schaller, E. L., Ray, T. L., Lopes, R. M. C., & Stofan, E. R. (2011). Rapid and extensive surface changes near Titan’s equator: Evidence of April showers. Science, 331, 1414–1417.

Turtle, E. P., Perry, J. E., McEwen, A. S., DelGenio, A. D., Barbara, J., West, R. A., Dawson, D. D., & Porco, C. C. (2009). Cassini imaging of Titan’s high-latitude lakes, clouds, and south-polar surface changes. Geophysical Research Letters, 36, L02204.

Wall, S., Hayes, A., Bristow, C., Lorenz, R., Stofan, E., Lunine, J., Le Gall, A., Janssen, M., Lopes, R., Wye, L., Soderblom, L., Paillou, P., Aharonson, O., Zebker, H., Farr, T., Mitri, G., Kirk, R., Mtchell, K., Notarnicola, C., Casarano, D., & Ventura, B. (2010). Active shoreline of Ontario Lacus, Titan: A morphological study of the lake and its surroundings. Geophysical Research Letters, 37, L05202.

Wye, L. C., Zebker, H. A., & Lorenz, R. D. (2009). Smoothness of Titan’s Ontario Lacus: Constraints from Cassini RADAR specular reflexion data. Geophysical Research Letters, 36, L16201.

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